Thermal Defocus calculator
Focal shift from thermal expansion and dn/dT of the lens material.
Category: Optomechanics
Formula: f′/f = (1 + α·ΔT)·(n − 1)/(n − 1 + (dn/dT)·ΔT) (≈ 1 + (α − (dn/dT)/(n−1))·ΔT)
Inputs
| Input | Parameter | Unit | Default |
|---|---|---|---|
| Focal length | f | mm | 100 |
| Index n | n | 1.517 | |
| α (CTE) | alpha | 10⁻⁶/K | 7.1 |
| dn/dT | dndT | 10⁻⁶/K | 3 |
| ΔT | dT | K | 20 |
Result with the default inputs
- Δf: 0.0026 mm
- Δf / f: 25.9428 ppm
Open it with your own values: put each input in the address as a parameter, for example /calculators/thermal-defocus?f=100. A unit other than the listed one goes in <parameter>_unit.
Read more: Wikipedia: Thermal lensing